Ferromagnetic particles generally used for a magnetic recording medium can be classified as follows.
(1) .gamma.-Fe.sub.2 O.sub.3 PA1 (2) Fe.sub.3 O.sub.4 PA1 (3) Intermediate product between .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 (Bertholide oxidation product) PA1 (4) Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 or Bertholide oxidation product PA1 (5) Co-coated .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 or Bertholide oxidation product PA1 (6) Ferromagnetic metal particles
Co-doped or Co-coated ferromagnetic particles (4) and (5) as mentioned above, which have a coericivity (Hc) of 500 to 800 Oe and are obtained by adding Co to ferromagnetic particles (1), (2) and (3) having a coericivity of 300 to 500 Oe, have been in increased demand with the increase of high density recording. Further, ferromagnetic metal particles (6) having a higher coercive force (Hc=1000 to 2000 Oe) have also been used. However, Co-added ferromagnetic particles (4) and (5) are not practically preferred because they change their characteristics with the passage of time (e.g., deterioration of erasure effect of signals, transferring, etc.), which is believed to be caused by diffusion of Co.sup.2+ ions. This phenomena becomes particularly noticeable when these particles are used at high temperature and high humidity. On the other hand, ferromagnetic metal particles (6) are not preferred because the saturation magnetization of ferromagnetic metal particles decreases with oxidation and ferromagnetic metal particles readily ignite, which can cause problems during the manufacturing process.